Many of today's manufacturing and inspection processes require precision dimensional measurements to ensure quality control and successful job completion. Displacement measuring "linear" interferometer systems capable of providing sub-micron linear displacement measurements are frequently employed in these applications.
Linear interferometers are simple in nature and serve a practical use when it is desired to measure the linear displacement of an object along a single axis. Linear interferometers typically combine a laser light source and interferometer with an optical target to obtain a displacement measurement. The laser light source provides a collimated beam of light which is directed towards a single reflecting element forming the optical target. The reflecting element, which is typically a prism, directly receives the collimated beam of light and reflects it back to the interferometer where it provides an optical reference as to the targets mechanical location along a single axis.
Linear interferometers are applied to a number of measurement tasks. One such task includes their use in manufacturing processes to provide precise feedback control of machine tool movements. In these systems the linear interferometers are imbedded into the machine tools and set to interact with certain optical targets mounted on critical reference surfaces of the machines. Typically these systems use one light beam per axis of movement. For example, a machine which moves along three axes will use three separate measurement beams to measure the machines displacement. Each light beam provides a measurement of the distance from the interferometer to the optical target. The exact displacement of the machine tool is then determined by the combined displacement information from the three axes.
Linear interferometer systems are also used to provide precise three dimensional measurements for the inspection and characterization of large surfaces, i.e., those found on turbines for hydraulic and nautical applications and mirror blanks for large telescopes. The systems utilized in these applications are known as "laser trackers" and consist of a linear interferometer system in which the laser light beam can be directed angularly from a fixed point towards the optical target. The optical target, once again, provides a means for directing the light back to the interferometer and serves as an optical reference point for a mechanical location. In its operation, the optical target is initially stationed in a precisely determined reference location and then removed by hand and placed against the surface of the object to be measured. As the optical target is moved by hand, the laser tracker beam follows the target using feedback control until it is placed at the point on the surface to be measured. The linear displacement of the target on the referenced location is then determined by the linear interferometer. The angular displacement of a target from the referenced location is determined by encoders on the optical system that directs the beam to the target via feedback control.
While these and other interferometer systems have enhanced inspection processes and proven essential in enabling the growth of the ultra-precision machine tool industry, they are not without their limitations. One key limitation lies within the fact that a linear displacement measurement alone fails to provide a complete picture of the true motion of the optical target. Ideally, one would seek to know all six degrees of freedom in which the target is moving, i.e., the three translational and three rotational degrees of freedom, and not just the movement along a single axis. The current linear interferometer systems are unable to adequately provide these measurements without compromising the original measurement of the translational displacement of the object along its measuring beam's optical axis.
Considering the examples described above, this limitation severely affects the ability of the manufacturing and inspection processes to function at their full potential. For example, machine tools, while designed to move in exact straight lines along their individual axes, always exhibit angular, translational, and rotational variations from perfect straight line movement. As manufacturing tolerances have become tighter over time, these errors have limited the ability of manufacturers to meet the manufacturing tolerances with their existing machines. In many cases, investing in more rigid and straighter machines is the only solution for achieving the tighter tolerance requirements.
It is recognized, however, that deviations from true straight line motion along the machine tool axis is tolerable if adequate measurement and compensation exist. This is typically achieved by a process called "machine tool error mapping" and may involve the use of several measurement systems to characterize repeatable machine tool errors caused by motion activities. Non-repeatable motion errors, however, (e.g., errors resulting from loading variations which are not reproducible from case to case) are not accounted for in the error mapping process and typically lead to errors in manufacturing.
As for linear interferometers utilized in inspection processes, the current systems are limited, once again, by the fact that they are only capable of measuring the linear displacement of the optical target along a single axis using one optical beam. Moreover, they fail to provide the capacity to monitor additional degrees of motion, i.e., pitch and yaw. These systems are also not capable of measuring the slope of the surface itself, thus resulting in an increase in the number of measurement points necessary to fully characterize the shape of the surface. Surfaces which are hidden (i.e., hidden behind obstacles such that they are not reached by the laser beam) also remain unmeasured by the current systems.
The need for machine tool systems having multi-axis measurements is well known and attempts are being made to address those needs by various interferometer manufacturers. For example, passive (i.e., consisting only of optical elements and no electronics) interferometric "targets" are manufactured to address angular measurement needs. However, these targets only provide one axis of angular measurement for a single optical beam and result in deteriorated displacement information. Another manufacturer provides a multi-axis "active" (i.e., contains photo detectors and electronics) target that monitors five degrees of motion, namely, displacement along the optical axis, and x and y translation perpendicular to the axis, pitch and yaw. Unfortunately, this system requires more than one optical beam and connection back to the laser source and interferometer system via wires. Consequently, these systems are not intrinsically compatible with the lasers and optical components used in conventional single beam linear interferometer systems.
U.S. Pat. No. 5,249,033, issued to the same inventor hereto and incorporated herein by reference, discloses a novel approach for aligning optical reflectors utilizing a method which allows the extraction of three-axis alignment information from a single optical beam. The development of a single optical beam multi-axis (displacement, pitch and yaw) interferometer measurement system was first developed to solve the difficulties associated with aligning, optical systems which employed aspheric (or non-spherical) reflecting, surfaces. The disclosed system provides for the use of a corner cube retroreflector to align a parabolic mirror segment to a fixed interferometer utilizing a single diverging optical beam provided by a lens. The lens is contained as part of the interferometer and causes the beam of the interferometer to converge to a focus. The paraboloidal segment is placed in an alignment such that the focus of the paraboloid is very near to the focus of the light from the interferometer lens, and such that a line passing through the center of the aperture of the paraboloidal segment and through the vertex of the parent paraboloid is parallel to the horizontal reference plane of the measurement system. Rotation about the axis formed by this line is defined as pitch. The corner cube retroreflector reflects the light from the paraboloidal segment back into the segment and thence into the interferometer where an interferogram is generated. The interferogram produced by the combination of the interferometer, lens, parabolic reflector, segment and corner cube retroreflector contains unique information about the pitch, yaw and linear displacement of a parabolic reflector segment relative to the interferometer. An electro-optical system interprets this information and implements it in an automated alignment system which makes certain adjustments to the position of the paraboloidal segment until the focus of the lens and the focus of the segment are co-located. The paraboloidal segment is moved relative to the interferometer and corner cube, which remain fixed with respect to each other.